Boron carbide (also referred to here as B4C) is the third hardest material next to diamond and cubic boron nitride. Combined with its low theoretical density (2.52 g/cm3), B4C is the premier material for personal armor-typically in the form of front and back flat plates which are bonded to a polymer backing and used as ballistic inserts in flack jackets. B4C is also used for nuclear shielding applications because of boron's high neutron absorption cross-section. In addition, B4C is used in particulate form as an abrasive, and as a nozzle material for slurry pumping and grit blasting because of its excellent abrasion resistance.
Effective ballistic armor materials must have very high hardness combined with high fracture toughness. When a high-velocity projectile makes contact with the surface of a ballistic material such as B4C, a compressive shock wave extends hemispherically from the point of contact, generating tensile, tangential stresses which cause radial cracks that emanate from the point of contact. These tangential stresses tear open cracks, preferentially at the site of pores and fissures. Therefore, ballistic performance of B4C improves with decreasing porosity, i.e. with increasing fired relative density.
Achieving near-theoretical density has required gang-hot pressing (stacked parts under pressure). Hot pressing does not allow for the cost effective fabrication of complex shapes. For example, the fabrication of form-fitting body armor parts would require machining after the hot pressing process, which is expensive and technically difficult.
Complex shapes (including form-fitting parts) are possible with pressureless sintering. According to the prior art, additives such as carbon, SiC, Al2O3, TiB2, AlF3 and W2B5 have been used as sintering agents in pressureless sintering to increase the sintered density. However, second phases due to the agents often have deleterious effects on the mechanical behavior of B4C.
The best known sintering agent for B4C is carbon. According to one prior art method, phenolic resin is used as a source of carbon. The carbon from the phenolic resin is distributed around the B4C particles, and also serves as a pressing agent.
Relative densities up to 98% have been obtained using carbon as a sintering agent. Carbon, when used as a sintering agent in pressureless sintering, however, promotes undesirable secondary phases and materials such as graphite which adversely affect the mechanical properties of the B4C.
Pressureless sintering of B4C without sintering agents has been difficult. Schwetz et al. in U.S. Pat. No. 4,195,066 cites to studies in which B4C has been pressureless sintered at near melting temperatures. However, the resulting material suffered in one study from low relative densities, and in the other study from poor mechanical properties compared to materials produced by hot pressing. In addition, Schwetz et al. noted that because the process required reaching close to the melting temperature of B4C it impaired the dimensional stability of the specimens.
In U.S. patent application Ser. No. 10/867,442 (assigned to the assignee of the present application) it is disclosed that limited densification pressureless sintering without sintering agents may be due to the presence of B2O3 coatings on B4C particles. It is further disclosed that the vaporization of B2O3 coatings permits direct B4C-B4C contact, and a corresponding surge in densification between 1870 and 2010° C. The loss of B2O3 coatings was implied by weight loss measurements.
Briefly, according to the disclosure of Ser. No. 10/867,442, to remove B2O3 coatings, B4C green body specimens are heated at a temperature between 1100° C.-1400° C. in a furnace and in the presence of a flowing He—H2 gas mixture. Prior to pressureless sintering, hydrogen is fully purged from the furnace chamber before continued heating. Otherwise, it is believed, hydrogen residing in interstitial locations within B4C particles facilitates increased evaporation/condensation coarsening of B4C, and consequently lower final densities. To purge hydrogen, the specimens can be soaked in He or held in vacuum for a period of time prior to pressureless sintering.
Specifically, the following method is taught in Ser. No. 10/867,442. After driving B2O3 out, the specimens are heated in the presence of He at a heating rate in the range 50 to 150° C./minute to a soaking temperature selected from the range 2300 to 2400° C., and held at the soaking temperature until the shrinkage rate is about 0.005%/minute. Using this method, specimens were pressureless sintered to as high as 96.7% RD.
In addition, Ser. No. 10/867,442 teaches that pressureless-sintered specimens can be further densified through hot isostatic pressing. The components so densified reached RD values above 99% when pressed under 310 MPa of gas pressure.
Through further study, it has been found that over the temperature range 1870-1950° C., particle coarsening occurred due to evaporation and condensation (from small to large particles) of rapidly evolving oxide gases (e.g. BO and CO), weight loss and particle/grain coarsening, stalled between 1960 and 2010° C., and resumed thereafter, concurrent with slowed densification up to about 2140° C. The resumption of weight loss and particle/grain coarsening, corresponds to evaporation and condensation of B4C (or its molecular fragments), a coarsening mechanism typical of such covalently-bonded solids. Above 2140° C. accelerated sintering occurred, which was projected to be caused by non-stoichiometric volatilization of B4C that left carbon behind. The carbon is believed to accelerate sintering through enhanced grain boundary diffusivity, i.e. activated sintering, and inhibiting grain growth to keep diffusion distances relatively short.
It was further found that rapid heating through the range 1870-1950° C. left less time for oxide-facilitated particle coarsening to take place (if oxide had not previously been removed by a lower temperature H2/He treatment), and through the range 2010-2140° C., minimized the time over which coarsening could occur by evaporation and condensation of B4C. Rapid heating brought comparatively small, high surface energy particles into an elevated temperature range, over which (activated) sintering was rapid relative to coarsening. Thus, rapid heating was found to be preferred to avoid particle coarsening.